Branched nanowires, also known as nanotrees, are versatile building blocks that increase achievable complexity in 3D architectures. Developing techniques capable of enhancing directed growth of nanotrees is essential to fully realize their potential for nanodevices. Nanotrees often serve as distributed conductors within a device's active volume, and such percolation networks are applicable in water splitting or photovoltaics. Attaining the electrical connections between adjacent nanowires required for these applications can be accomplished by joining branches, forming 3D networks of nanowires. Unfortunately, producing interconnections between adjacent nanowire branches has, to date, required epitaxial growth, and periodically patterned catalyst seed layers.
Nanotree growth is typically achieved via secondary growth of branches on the side of a vertically aligned nanotree trunk. Vapor-liquid-solid (VLS) growth has been commonly used to direct nucleation and growth of nanotree trunks and branches, although dislocation driven trunk growth has also been demonstrated. Recent progress in controlling branch morphology includes adjusting branch density along the nanotree, placing branches selectively along the height of the trunk, growth of hyperbranched structures, and modulating the branch diameter during growth. Controlled branch placement on selected sides of the trunk has remained elusive. The capability to place branches on a selected side of the trunk would allow for improved control over connectivity in nanowire networks.
Indium tin oxide (ITO) is a transparent conductive oxide (TCO) which is commonly used in organic light emitting devices, flat-panel displays and touchscreens, and organic solar devices. The organic photovoltaic (OPV) community has shown interest in nanostructured ITO films for high surface area electrodes. OPVs are often limited either by light absorption or charge extraction. High surface area electrodes decouple these two limiting factors by allowing for increased absorption while maintaining short charge extraction distances. Conductive pathways throughout the active layer allow the active layer thickness to be increased beyond the exciton diffusion length, motivating development of nanostructured electrodes.
Nanostructured electrodes with increased surface areas have been fabricated using anodic aluminum oxide templates, organic vapor phase deposition and vapor-liquid-solid (VLS) growth of high aspect ratio nanowires. VLS is a crystal growth technique that results in high surface area nanostructures that grow primarily in one dimension. ITO nanowire films can be grown with a self-catalyzed VLS growth mode accessible at elevated substrate temperatures. Beyond applications as high surface area electrodes, ITO nanowires are applicable as gas sensors, protein molecule sensors and UV light sources.
During physical vapour deposition of ITO, self-catalytic indium-tin alloy liquid droplets form on the substrate's surface if the substrate is heated above the alloy's eutectic point. The liquid droplet collects impinging vapour atoms as it has high sticking coefficient (the ratio of atoms adsorbed to the total number of atoms incident on the surface) with respect to the substrate. Nucleation occurs preferentially at the droplet-substrate interface, restricting the nanowire's lateral growth. This results primarily in crystal growth in one dimension, with the primary axis (trunk) normal to the liquid-solid interface.
ITO nanowires exhibit distinctive growth of branches orthogonal to the axial growth direction of the nanowire, reflecting the cubic bixbyite crystal structure of indium oxide. The branching mechanism produces high surface area nanowires, which may improve electrical access to OPV active device layers. ITO nanowire growth has been reported with a large range of substrates, flux rates, vapour incidence angles and deposition techniques.
The vapor-liquid-solid (VLS) mechanism, whereby growth proceeds by precipitation from a liquid catalyst that concentrates the surrounding vapor, has been used to grow crystalline nanowires. Branched nanowires (or nanotrees) can be formed by placing catalytic droplets on the sides of nanowires during growth via a stochastic or engineered process, enabling bottom-up fabrication of complex three-dimensional architectures. Control over intra-wire morphology has also been investigated, with several groups reporting variations in nanowire diameters. Givargizov and others suggest an unstable model of self-oscillations based on droplet contact angle and surface roughness driven by droplet supersaturation. Most of these reports attribute rippled (or bamboo) nanowire structures to this self-oscillatory growth model. However, others have demonstrated discontinuous diameter changes through annealing-driven catalyst migration, switching between different crystal cross-sections during growth, and segmented nanowire morphologies controlled by carrier gas pulsing.
Recently, a geometrical modification of VLS growth through glancing angle deposition (GLAD) was developed. For the self-catalyzed ITO system, this technique (VLS-GLAD) provides additional control over nanowire diameter, spacing, and branching behavior.
Glancing-angle-deposition (GLAD) is a physical vapor deposition technique that utilizes a spatially modulated vapour flux to control thin film morphology.